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Author: Saptarshi Chatterjee & Neloy Kumar Chakroborty

Physiology and Immunology of Bats – A Special Reservoir of Deadly Viruses

The Dreadful Myth of Bram Stoker’s ‘Dracula Transforming into a Bat’ seems to be a Reality when Considering the Physiology and Immunology of Bats – A Special Reservoir of Deadly Viruses

Student contributors (Authors): Ms. Animikha Ghosh, Ms. Shrestha Sengupta, Students of B.Tech Biotechnology, Semester-IV, Department of Biotechnology

The vampire fear and the prevailing superstitions in societies:

It’s been one hundred and twenty-three years passed after its publication, the Gothic vampire tale; ‘Dracula’, written by the Irish author Abraham (“Bram”) Stoker is still staggeringly popular worldwide among all age groups and professions. The careful and detailed creations of horror-plots of the original story have terrified the readers and elicited nightmares from the 19th century till today. The radical transformation of ‘Dracula’ into a bat, as depicted by Stoker, contributes a lot to the thrill of the story but unfortunately, it has also strengthened some of the prevailing superstitions, misinformation and credulous beliefs of the human minds about the bats – such as bats as ‘blood-sucking evil creatures’ or ‘symbols of devilish activities’. Some oriental and occidental societies still believe that bats drink human blood.

Bats are largely insectivorous and frugivores though some drink mammalian blood too:

Bats belong to the order Chiroptera, which is the second-largest species-rich mammalian order with over 1300 species that are distributed in every other continent except Antarctica (bats comprise >20% of the classified mammalian species on earth). The order Chiroptera has two sub-orders, Yinpterochiroptera (comprises both families of megabats and microbats) and Yangochiroptera (the remaining microbat families). Bats of these suborders have diverse food habits but a majority of the members of the microbat families are insectivorous, i.e. they feed on moths, mosquitoes, beetles, etc. The consumption of insects can be as high as more than 500 small-sized insects per hour for the single little brown bats. The bulk of the megabats and other microbats however are fruit-eaters or frugivores – eating fruits (bananas, mangoes, figs, and dates), seeds, and pollen from the flowers. Also, some bats feed on birds, frogs, lizards, and other bats, and very few even drink the blood of other mammals. This last group of bats, popularly known as ‘Vampire Bats’, belong to the subfamily Desmodontinae and are native to the Americas, ranging from Central to South America. They represent only three species (Desmodus rotundus, Diphylla ecaudata, and Diaemus youngi) among the 1300 bat species found globally. The commonly seen vampire bat or Desmodus rotundus solely take blood as their food, albeit it’s not the human blood but they feed on the blood of cows, sheep, and horses. Therefore, the prevailing concepts about bats as a human bloodsucker and this is a ‘morph of Vampire’ are biologically invalid.

Bats harbor diverse viruses and high-profile zoonotic viruses – A serious threat to public health:

Before going any further, it is necessary to define what a zoonotic virus is. According to the World Health Organization (WHO), any disease or infection that can be naturally transmitted from a vertebrate animal host (fishes, amphibians, reptiles, birds, and mammals) to humans is defined as a zoonotic disease, and therefore, any viral pathogen that causes a zoonotic disease is called a zoonotic virus. The novel coronavirus, SARS-CoV-2, causing the current COVID-19 pandemic is an example of a zoonotic virus that probably comes from particular species of bats to humans via a second animal host (probably the pangolin). Bats harbor a huge number of zoonotic viruses with the highest number of viruses found to be hosted per bat species among the mammal orders. Estimations have also revealed that there are about 9.8% of the bat species that act as the zoonotic hosts of these viruses (members of more than 15 families of viruses) to cause >25 unique zoonotic diseases. Virological, epidemiological and molecular studies have clearly established the links between the bat species as natural reservoirs of high-profile zoonotic viruses and the emergence and re-emergence of viral epidemics and pandemics in human populations – they include the fatal diseases caused by rabies (transmitted by the vampire bats through carnivores to humans) and other related lyssa viruses, Nipah virus infection with fatal encephalitis (case fatality rate >70%) and diseases caused by other Henipaviruses such as Hendra viruses, Marburg virus disease with hemorrhagic fever (case fatality rate is between 25% and 100%), Ebola haemorrhagic fever (case fatality rate is between 25% and 90%), and the coronaviruses causing acute respiratory syndromes such as the novel SARS (Severe Acute Respiratory Syndrome) coronavirus (case fatality rate of 10%), the novel MERS (Middle East respiratory syndrome) coronavirus (case fatality rate of 34.4%) and the novel SARS-CoV-2 (case fatality rate of 6.2% till 29.05.2020) that is causing the current COVID-19 pandemic.

Apart from the highly pathogenic viruses, the huge viral diversity found in bat species also includes viruses which are not yet reported to be transmitted from bats to other animal hosts or causing human diseases, and even for some of them, it is still unclear whether bats are one of their important natural reservoirs such as particular alphaviruses, flaviviruses, and bunyaviruses.

Bats are special reservoir hosts of viruses owing to their unique physiology and immunology – The current status of knowledge:

Bats are the reservoir hosts of a plethora of zoonotic viruses partly because of the lifestyles of the different species to live together in overlapping geographic regions at the same time (a phenomenon called sympatry) which results in increased sharing of viruses between the species due to profound inter-species transmission. Though bats act as the natural reservoirs of viruses that are virulent for other animals and humans, rarely bats die or show clinical symptoms of these viral diseases. The maintenance of high viral diversity with low virulence in bats has led to many speculative hypotheses on the functional mechanisms that bats use to regulate viral replications more efficiently compared to other mammals. One such hypothesis states that evolution of flight is associated with high viral-tolerance in bats as the increased metabolic rate and the higher body temperature of bats during the flight together probably played the role of a general immune booster on an evolutionary timescale to select for the mitigating of virulence of the infecting-viruses at the cost of fostering viral diversity in the bat populations. This eventually enables bats to tolerate a greater diversity of virulent viruses by metabolic and thermal activations of the different branches of the immune system, such as the components of the innate and adaptive immune systems. Support for this hypothesis came from the following evidences – substantial increase in the rate metabolism (15-17 fold) in bats during flight compared to the resting state and this increase is way higher than the increase in metabolic rates of a flying bird (2 fold) or a sprinting rodent (7 fold), laboratory strains of mice inbred for higher metabolic rates surmount significantly stronger and specific immune responses compared to the mice bred for lower metabolic rates when challenged with a specific antigen,  evolution of flight behavior in bats was accompanied by genetic changes linked with the crucial requirement of repairing DNA damages due to increased rate of metabolism, elevation of core body temperature, typically found during mammalian fever (38oC – 41oC), in different bat species during their flights (e.g. Rousettus aegyptiacus: 38.2oC – 41.2oC, Eptisicus fuscus: 41oC, Eumops perotis: 37.8oC – 39.3oC, Carollia perspicillata: 40.2oC), and lastly the possible anti-viral innate and adaptive immune responses that can be induced in bats by high body temperature (enhancement in leukocyte mobility and interferon production, increased antibody production and cytokine responses, and increased T-helper cell responses etc.). These results, in lights of the physiology and immune system of bats and viral diversity, strongly suggest that bats are special reservoir hosts of zoonotic viruses owing to their flight behavior that escalates the immune responses on a daily-basis to possibly promote a form of evolution in the bat-borne viruses which enable them to sustain infections in bats without causing the diseases.

More direct evidence has disclosed two unique anti-viral mechanisms operating in the bats which are never reported previously in any other model system; these are respectively the chronic but regulated inflammatory responses and the antibody-level independent protective immunity. The innate immune system of bats responds to viral infections through type-I interferon (IFN) and the IFN-stimulated genes or ISGs, which eventually leads to the expressions of antiviral and pro-inflammatory cytokines. However, unlike humans, bats can regulate the high-levels of virus-induced pro-inflammatory responses, and thereby limit the tissue damage, by novel mechanisms (one of them is repressing the expressions of cytokine genes) but at the same time preserving the IFN-responses persistently to check viral replication. Interestingly, though bats have all the major subclasses of antibody (IgA, IgG, IgM, and IgE), under conditions of a challenge with the viral antigens, they expressed low titers of antibodies, even sometimes below the threshold of seropositivity, although the animals became seronegative at that time with no detectable viremia or viral shedding. The antibodies isolated from the bats, after they became seronegative, also didn’t show any neutralizing reaction to the antigens. This peculiar behavior of bat antibodies, of not showing viral neutralization, together with the persistent pro-inflammatory responses and the presence of a higher number of germline genes for antibodies than the humans confirm that the immune systems of bats are highly adapted to withstand the chronic load of diverse viruses, including the highly virulent ones, and possibly have substantial functional differences than that of the humans.

Our responsibilities to avoid further infections from the bats:

It is logical to state that virulent viruses although don’t make the bats sick but when transmitted (zoonotic events) from the bats to other animals, especially to human, whose immune pathways are unprepared to cope up with these viruses, can cause fatal diseases. The deadly epidemics and pandemics in humans, such as Ebola, SARS, MERS, Nipah, and the current COVID-19 are some of the blatant examples of human disasters in which bats played the villains by transmitting these viruses. However, accusing bats is nonsensical and is merely a way to escape our misdemeanor of interfering with the bats that have nucleated these catastrophes. Thus, we have to take vital responsibilities at this hour of crisis to avoid further bat-borne epidemics or pandemics, which include (by the WHO-guidelines) avoidance of contacts with the bats (bats are peri-domestic, so they often live in human residence), preserving their habitats, taking measures to check fruits for bat-inflicted perforations before selling them in the markets, planning agricultural land encroachment without destroying the forests, implementing re-forestations, and most importantly stop slaughtering bats for food. In the end, we have to remember that bats are neither our enemies nor vampires, but in reality, they are our true friends who are involved in maintaining the balance of our ecosystem and pollinating important crops to sustain our lives.

References:

  1. Simmons, N.B. (2005). “Order Chiroptera”. In Wilson DE, Reeder DM, editors. Mammal species of the world: a taxonomic and geographic reference. JHU Press; 2005.
  2. Greenhall AM. Natural history of vampire bats. CRC Press; 2018 May 4.
  3. Hawkey C. Plasminogen activator in saliva of the vampire bat Desmodus rotundus. Nature. 1966 Jul;211(5047):434-5.
  4. Han BA, Kramer AM, Drake JM. Global patterns of zoonotic disease in mammals. Trends in parasitology. 2016 Jul 1;32(7):565-77.
  5. Olival KJ, Hosseini PR, Zambrana-Torrelio C, Ross N, Bogich TL, Daszak P. Host and viral traits predict zoonotic spillover from mammals. Nature. 2017 Jun; 546(7660):646-50.
  6. Calisher CH, Childs JE, Field HE, Holmes KV, Schountz T. Bats: important reservoir hosts of emerging viruses. Clinical microbiology reviews. 2006 Jul 1;19(3):531-45.
  7. Banerjee A, Baker ML, Kulcsar K, Misra V, Plowright R, Mossman K. Novel insights into immune systems of bats. Frontiers in immunology. 2020 Jan 24;11:26.
  8. Luis AD, Hayman DT, O’Shea TJ, Cryan PM, Gilbert AT, Pulliam JR, Mills JN, Timonin ME, Willis CK, Cunningham AA, Fooks AR. A comparison of bats and rodents as reservoirs of zoonotic viruses: are bats special?. Proceedings of the Royal Society B: Biological Sciences. 2013 Apr 7;280(1756):20122753.
  9. Brierley L, Vonhof MJ, Olival KJ, Daszak P, Jones KE. Quantifying global drivers of zoonotic bat viruses: a process-based perspective. The American Naturalist. 2016 Feb 1;187(2):E53-64.
  10. O’shea TJ, Cryan PM, Cunningham AA, Fooks AR, Hayman DT, Luis AD, Peel AJ, Plowright RK, Wood JL. Bat flight and zoonotic viruses. Emerging infectious diseases. 2014 May;20(5):741.

Food Safety & Quality Control: Microbiological Aspects & Scopes

Food: Any substance that can be consumed to obtain energy and contains nutritional as well as aesthetic value is termed as food. Food is considered as the principal requirement for all forms of a living creature to survive and reproduce on the planet earth.

Food Security: According to the ‘World Food Summit’ of 1996, food security is defined as “when all people at all times have access to sufficient, safe, nutritious food to maintain a healthy and active life”. Thus food security not only assures a sufficient quantity of food but also the quality of food.

Food Safety: Food Safety and Standard Act 2006 defines food safety as the “assurance that food is acceptable for human consumption according to its intended use”. Also, the WHO considers global prevention, detection, and response to the threats associated with unsafe food as ‘Public health concern’. 

Food Safety& Microbiology: The greatest challenge to food safety is microorganisms that account for more than 200 diseases caused by bacteria, viruses, protozoa, and fungi. Recent-time statistics have shown that nearly 420,000 people die every year due to foodborne illnesses. The case is even severe in children, especially those who are below 5 years, which alone counts for 40% of the total burden caused by foodborne diseases. Foodborne illnesses are primarily classified as food infection and food intoxication. While in the first case, live microorganism enters the body, multiplies and damages host tissue, the later can cause damage by the production of toxin in food, despite being absent in the body.

The Economics of Food Borne Diseases: The loss of productivity due to foodborne illness is estimated to 95.2 billion USD per year in the low and middle-income countries while the annual cost of treatment was estimated at 15 billion USD, as presented in the annual report of the World Bank in 2018.

Quality Management System: Quality Management System or QMS is a strategic direction that enables an organization to achieve the targets in terms of the fulfilments of the pre-defined policies. However, for the food industry, it also includes improving the overall quality of food safety & hygiene in compliance with the regulatory standards. Thus the targets of QMS, apart from the fulfillment of organizational aspiration and adherence to its policy, also include the productions of ‘zero risk food’, otherwise known as ‘safe food’ for its customers.

Health Hazards Associated with Food: Three major hazards may contaminate food and lead to a breach of food safety; these are:

  1. Physical Hazard: Presence of dirt, dust, metal, hair, etc. that contaminate food
  2. Chemical Hazard: Presence of pesticides, heavy metals, and allergens, etc.
  3. Biological Hazard: Presence of microorganisms (mainly)

Among the three, the first two (i.e. Physical and Chemical) hazards can be eliminated during the stages of raw material screening and processing, however, controlling microorganisms is difficult as food serves as nutritional supports for the organisms and help them to grow. As a result, most of the food spoilages are also attributed to microbial spoilages apart from the factor like self-degradation by chemical reactions.

Quality Control: The food produced in an industry must be safe and free of any of the hazards mentioned earlier. Quality Control (QC) is a part of the QMS and it is a reactive process aiming to identify and rectify defects in food products, in terms of hazards. The QC team of the food and beverage industry has skilled microbiologists and chemists to perform the analyses of food products and adhering to the Quality Assurance (QA) parameters and the regulatory guidelines.

Roles of the Microbiologist: Microbiologists play vital roles in the food industry, especially in controlling the quality (QC) of food products. The job responsibilities of a Microbiologist include:

  1. Routine Quality Control: Regular monitoring of the raw materials, process intermediates as well as the finished products in terms of microbiological quality.
  2. Risk Analysis, Assessment, Communication & Management: Any substance, action, and method that increases the probability of adverse effects on the health of the consumers or leads to food hazards are termed as risk. The scientific process of analyzing risk using tools like Hazard Analysis and Critical Control Point (HACCP) followed by an assessment of the grade of risk with proper communication, to manage the process through scientific intervention, is one of the major job-roles of the microbiologists working in the food industry.
  3. Environmental Monitoring: The safety of the food largely depends on the environment of food production and subsequent packaging. Therefore, a safe environment is a prerequisite to maintain the sterility of food products or at least a safe level for ensuring food safety. Environmental monitoring of the storage area, production area, instruments and packaging area, etc. are carried out to determine the microbial load, adhering to the policy and regulatory guidelines.
  4. Food Hygiene: The roles of microbiologists also include the maintenance of all conditions and necessary measures to ensure the safety and suitability of food at all stages of the food production and supply; commonly referred to as food hygiene.
  5. Food Inspection & Surveillance: Microbiologists are required to conduct routine examinations, empowered by the regulations of food products and system, to confirm the regulatory compliance and adhere to the norms associated with food safety. This functionality is known as food inspection while continuous monitoring is termed as food surveillance.
  6. Quality Management System (QMS): Microbiologists, as part of the entire management, are involved in designing the process and formulating the ‘Quality Policy Document’ to be followed. Routine monitoring of quality and survey based on HACCP are important job roles.
  7. Food Regulatory Compliance: Microbiologists are involved in regulatory affairs in maintaining the safety of food, adhering to the regulatory guidelines.

Analytical Skill-Sets required for getting QC-Jobs: Microbiologists, aspiring Quality Control jobs must develop these analytical skills along with the domain knowledge to emerge as front-runners in securing such jobs:

  1. Basic Microbiological Skills: Media Preparation, Sterilization, Maintenance of Aseptic Condition, Strain Maintenance, Pure Culture Techniques, Staining Techniques, etc.
  2. Applied Microbiological Skills: Environmental Monitoring, Isolation and Enumeration of Bacteria, Identification of Microorganisms.
  3. Analytical Instrumentation usage: High-Performance Liquid Chromatography (HPLC), Gas Chromatography (GC), Atomic Adsorption Spectroscopy (AAS), Enzyme-Linked Immunosorbent Assay (ELISA), etc.
  4. Quality Management: QMS, Food Audit, Regulatory Aspects, and Food Laws.

Regulatory Aspect of Food: Several food regulatory systems enforce Quality Assurance Plan for food safety. A few of them are:

  1. Good Agricultural Practices (GAP)
  2. Good Manufacturing Practices (GMP)
  3. Good Hygienic Practices (GHP)
  4. Good Laboratory Practices (GLP)
  5. Hazard Analysis and Critical Control Point (HACCP)
  6. International Standard Organization (ISO)

Many of these are directional and offer general guidelines to be implemented accordingly in the food industries. However, there are Mandatory standards in India that are to be implemented and monitored under various levels of the Ministry, Govt. of India.

Mandatory Standards: The Prevention of Food Adulteration (PFA) Act 1954 is mandatory to be adhered to by any food industry and is enforced in a three-tier system: Union (Govt. of India), State/UT, Local Bodies. In this regard, the central Govt. plays an advisory role in carrying out the statutory functions. This comes under the purview of the Ministry of Health and Family Welfare, Govt. of India. Other mandatory standards are:

  1. Fruit Product Order (FPO) 1955
  2. Meat Food Products Order (MFPO) 1973
  3. Consumer Protection Act 1986
  4. Agriculture Product and Marketing Act (AGMARK) 1937
  5. Edible Oils Packaging Order 1998
  6. Environment Protection Act 1986
  7. Essential Commodities Act 1955

Job Prospects: The posts, mentioned below, are exclusively offered to the trained microbiologists in various food industries, regulatory authorities, and Govt. functionalities, which include hospitals, municipalities, etc. Proper blending of scientific knowledge, technical and analytical skills, and ideas regarding regulatory affairs and quality management system thus can help students in securing jobs in these sectors.

  1. Quality Control Officer (Microbiology) in the Food and Beverage Industry
  2. Member, Quality Management System (Food Safety)
  3. Officer, Food Regulatory Affairs
  4. Government FSSAI, BIS, etc.
  5. Consultant: Food Safety and Quality Management
  6. Microbiologist: Food Laboratory
  7. Food Inspectors, Food Safety Officers
  8. Food Quarantine Officers
  9. Food QC Officer: Hospitals

References/ Further Reading: 

  1. FSSAI: https://www.fssai.gov.in
  2. Ministry of Food Processing Industries: https://mofpi.nic.in/Schemes/implementation-haccp-iso-22000-iso-9000-ghp-gmp-etc
  3. WHO: https://www.who.int/news-room/fact-sheets/detail/food-safety
  4. US FDA- HACCP https://www.fda.gov/food/hazard-analysis-critical-control-point-haccp/haccp-principles-application-guidelines
  5. CODEX ALIMENTARIUS- http://www.fao.org/fao-who-codexalimentarius/en/

 

 

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